1. Field of the Invention
Systems consistent with the present invention relate to an array antenna and, more particularly, to an array antenna system for rapidly producing a beam patterns in accordance with a communication direction of radio signals.
2. Description of the Related Art
Recently, in the radio communications, a communication scheme is being developed to transmit and receive radio signals over a short range. It is known in the art to employ an array antenna for such communications. Such a communication scheme is often used in millimeter wave communication. However, array antennas, along with their associated techniques, are used in other types of communication as well.
Typically, related art array antennas include a plurality of antenna elements. Beams radiated from the antenna elements are overlapped to obtain directivity, and it is desirable to have high main lobe gain, and a sufficiently low sidelobe level in order to avoid interference with other devices.
Since array antennas have directivity, a vertical angle of the main lobe (i.e., a tilt angle) is changed so as to transmit and receive radio signals in a specific direction. The vertical angle of the main lobe can be changed by tweaking a phase between signals fed to the antenna elements. By this means, the gain of the main lobe can be raised.
A related art method of tweaking the phase between signals is to adjust the phase of the signals received at the antenna elements using variable phase shifters. The antenna elements are thus provided with separate variable phase shifters respectively. The phase of the radio signal from the antenna element is tweaked by the variable phase shifter, to thus change the vertical angle of the main lobe. Whether the tilt angle is optimal is determined after the phase of every tiltable angle is adjusted.
In the related art method using the variable phase shifter, the tilt angle has to be changed by continually shifting the phase at the variable phase shifter until the optimum main lobe gain and the optimum sidelobe level are acquired. This method has a disadvantage in that it takes a long time to acquire an optimum tilt angle. Additionally, because each antenna element needs to include a separate variable phase shifter, the cost and the size of the antenna system are high, which is problematic. To address some of these shortcomings, a method using a single variable phase shifter has been suggested. However, the single variable phase shifter raises the sidelobe level, creating a disadvantage in that interference with other devices is increased.